Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360. This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode, and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are designed to have a thickness equal to .lambda./4 where .lambda. is the wavelength of the incident light source. They are supported a distance of .lambda./4 above, and parallel to, the substrate. Thus, when the deformable elements are unactivated, i.e., undeflected, the distance between their top surface and the substrate equals .lambda./2. Thus when light impinges perpendicularly to the surface of this surface the grating reflects light as a flat mirror. However, when a sufficient voltage (switching voltage) is applied between the deformable elements and the substrate, the resulting electrostatic force pulls a portion of the deformable elements down a distance .lambda./4 toward the substrate, thereby reducing the distance between the top this portion of the elements and the substrate to .lambda./4. Thus, light reflected from this portion of the deformable elements is out of phase with that from the substrate and a diffraction pattern is formed. Optical systems can intercept the diffracted light with output occurring only when the deformable elements are activated (i.e., pulled down). For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image. Furthermore, since gratings are inherently dispersive, this modulator can be used for color displays.
U.S. Pat. No. 5,677,783 by Bloom et al. discloses a modulator which obviate stiction between grating elements and underlying substrate.
One problem with the prior art modulator is that only a portion of the activated deformable elements deflect the desired distance .lambda./4, thereby limiting the diffraction efficiency of the modulator.